Various types of electro-optical scanning apparatus are known and are incorporated in a wide range of apparatus. In general, the scanning apparatus includes a light source which directly illuminates a surface to be scanned and a camera for sensing visible characteristics of the surface. Both the camera and the light source may be mounted on a movable base which may undergo translation in scanning relationship with a surface. Alternatively, the camera may remain stationary and the surface may be translated relative thereto. As a further alternative, both the camera and the scanned surface remain stationary and the scanning is only over the time dimension.
Normally scanning apparatus of the type described hereinabove uses a very high intensity light source in order to obtain an acceptable signal to noise ratio at the camera output. The use of such a high intensity light source involves a number of difficulties: A great amount of heat is generated, while at the same time, the high intensity of the light source requires nearly complete shielding in order to prevent injury to an operator. Thus forced air cooling systems are required to be provided in association with the light source.
Secondly, the high intensity of the illumination tends to produce shadows in the surface being scanned as the result of surface irregularities therein, resulting in incorrect scanning outputs. Thirdly, the great amount of heat generated by the light source may produce distortions in the surface being scanned and may even damage the surface. The above problems are particularly significant when a camera such as a CCD camera is employed, since overcoming them limits operation of the camera and does not enable the entire dynamic range of the camera to be employed. As a result, complex and expensive signal processing circuitry is required at the output of the camera to compensate.